US3172980A

US3172980A - Compressed air switch with multiple interruption

Info

Publication number: US3172980A
Application number: US35682A
Authority: US
Inventors: Thommen Hans
Original assignee: BBC Brown Boveri France SA
Current assignee: BBC Brown Boveri AG Germany; BBC Brown Boveri France SA
Priority date: 1959-06-13
Filing date: 1960-06-13
Publication date: 1965-03-09
Anticipated expiration: 1982-03-09
Also published as: GB928916A; DE1099039B; BE591806A; FR1259749A; CH372721A

Description

H. THOMMEN March 9, 1965 COMPRESSED AIR SWITCH WITH MULTIPLE INTERRUPTION 2 Sheets-Sheet l Filed June 15, 1960 INVENTOR ATTORNEYS 2 Sheets-Sheet 2 H. THOMMEN March 9, 1965 Filed June 13, 1960 INVENTOR HQJLS 7710! men mi -1 www ATTORNEYS United States Patent Oiice 3,172,980 Patented Mar. 9, 1965 3,172,9@ CMPRESSED AlR SWITCH WITH MULTTPLE ENTERRUPTIUN Hans rllhonnnen, linden, Switzerland, assigner to Aktiengesellschaft Brown, lios/eri da Ele, Baden, Switzerland, a joint-stock company Filed .lune 13, woll, Ser. No. 35,682 Claims priority, application Switzerland, lune l, i959,

z claims. (ici. 2te- Msi This invention relates in general to compressed air actuated electrical switching apparatus wherein the contacts which define the switching points are blasted with compressed air, and in particular to switches of the general type described which include a plurality ot power interrupting switching points and a voltage disconnecting or isolating switching point, all of which are connected electrically in series, and one of the power switching points being paralleled with a low-ohmic resistance. Another of the power interrupting switching points can also, if desired, be paralleled by a high-ohmic resistance for voltage control. The function of the power interrupting switching points is to satisfactorily disconnect the load current through the switching apparatus up to the highest circuit-breaking rating for which the apparatus is designed. The function of the voltage disconnecting switching point is to ensure voltage isolation between the input and output terminals of the switching apparatus during the entire period that the latter is in its disconnected state.

It is known to design air blast switches with a plurality of power interrupting switching points wherein such points are associated together in various numbers depending upon the magnitude of the operating voltage of the system to which the switching apparatus is applied, and also the magnitude of the load current which must be interrupted, each of these elements being designed in the same manner.

ln newer developments of air blast switching apparatus, it has ybecome necessary to further increase their circuit-breaking capacity. To this end, it has been known to provide two power interrupting switching points in series and to connect a low-ohmic resistance in parallel with one of the power interrupting switching points. A voltage disconnecting switching point is also connected in series with the two power interrupting switching points. ln operation, the power interrupting switching point which is paralleled by the low-ohmic resistance is opened rst thereby to divert the current flow through the resistance, and then the other power interrupting switching point opens in order to interrupt the current flowing through the resistance. The voltage disconnecting switching point is then opened and maintained in an open state so long as the open circuit condition is desired. The two power interrupting switching points can be allowed to re-close as soon as the voltage disconnecting switching point has been opened so that the circuit can be re-closed through the switching apparatus simply by re-closing the contacts at the voltage disconnecting switching point.

The compressed air required for actuating the power interrupting and voltage disconnecting switching points and for blasting the same with the compressed air in order to facilitate arc extinction across the switch contacts, is usually obtained from a common supply tank in which the air is maintained under the desired pressure. Where the switching apparatus is composed of a plurality of similar circuit breaking groups, it has already been proposed to sub-divide the pressure tank corresponding to the number circuit breaking groups, i.e. to associate each tank with one, or more, groups of power interrupting switching points of `both types, and with one voltage disconnecting switching point. The whole switch assembly thus becomes quite complicated, since each group requires separate air control valves for the switching. The supply of compressed air and the arrangement of the air control valves becomes even more complicated it the switching motion is different in the various switching points and if the high-ohmic resistances when used, also have separate auxiliary switching points to control their parallel connection with their associated power interrupting switching points, i.e. when the power interrupting switching points for example, close immediately after extinction of the arc, while the voltage disconnecting switching point remains open, and the auxiliary switching point for the high-ohmic resistances first closes at the instant the entire switching apparatus begins an opening function and then re-opens.

The object of the present invention is to provide an improved structural arrangement for switching apparatus ot the general type described which is more simple and economical to manufacture and which permits one to more easily assemble and adapt the various components to diiierent power and voltage requirements. In accordance with the invention, the improved structural assembly is characterized by two series connected power interrupting switching points of the air blast type, one of which is paralleled by a low-ohmic resistance, a voltage disconnecting switching point also of the air blast type in series with the power interrupting switching points, an auxiliary 'compressed air tank having built-in main valves for controlling admission of compressed air to the power interrupting switching points, at least one such switching point being electrically isolated from the auxiliary compressed air tank by a tubular insulator located on the tank.

The invention will become more apparent from the following detailed description of several practical embodiments and from the accompanying drawings wherein:

FlG. l is a fragmentary view of one embodiment of the invention, the switching structure essential to an understanding of the invention being shown partly in vertical central section and partly in elevation;

FlG. 2 is a View similar to FiG. l illustrating a modiiied embodiment of the invention;

FIG. 3 is a schematic circuit diagram showing only the arrangement of the power interrupting switching points and the voltage disconnecting or isolating switching points in an organization which comprises two similar switching units connected in series, each such unit being constituted by a series connected arrangement of two power switchin.fr points and a volta ge disconnecting switching point, and a low ohrnic resistance connected in parallel with a series arrangement of one power interrupting switching point of one switch unit and one power interrupting switching point of the other switch unit;

FIG. 4- is a view similar to FlG. l but showing a somewhat ditlerent embodiment; and

FIG. 5 is a View similar to FlG. 4 but showing a moditication of the latter.

With reference now to the embodiment illustrated in FIG. 1 it will be seen that the switch structure comprises an auxiliary compressed air tank l, a laterally extending hollow insulator 2 supported by tank 1 and containing a pair of normally closed, power interrupting,

switch contacts

2a, 2b, and a vertically upward extending hollow insulator 3 also supported by tank 1 and containing a pair of normally closed, power interrupting, switch contacts Ela, 3b. A pair of normally closed, voltage disconnecting, switching

contacts

5a, 5b is contained within an inclined hollow insulator 5 which is connected at its upper end with the upper end of insulator 3'. The two power

interrupting switching points

2a, 2b and 3a, 3b and the voltage disconnecting

point

5a, 5b are connected electrically in series. A low-ohmic resistance 6 is connected in parallel with the power interrupting switching point 3a, 3b and a high-ohmic resistance 4 can, if desired, be connected in parallel with the power

interrupting switching point

2a, 2b.

Within the auxiliary compressed air tank 1 is arranged the main control valve structure for the two power interrupting switching points. This comprises a casing 9 containing two compressed air operated piston-cylinder type valve units 7 and S disposed at right angles to each other. Unit 7 functions to control admission of compressed air from tank 1 into the hollow insulator 2 so as to effect separation of the

contacts

2a, 2b. Unit 8 functions in a similar manner to control admission ot compressed air from tank 1 into the hollow insulator 3' so as to effect separation of the contacts 3a, 3b.

The auxiliary compressed air tank 1 is supported atop a vertical hollow insulator column 15 through which compressed air from a suitable primary source of supply such as a tank (not shown) is introduced into the auxiliary tank 1, and a valve 10 serves to control admission of compressed air into casing 9 so as to effect operation of the piston-

cylinder units

7, 8 which, in turn, control admission of compressed air to the

switch contacts

2a, 2b and 3a, 3b.

The inclined insulator 5' which contains the voltage disconnecting

contacts

5a, 5b is supported by a vertical hollow insulator column 14 through which compressed air is fed to the hollow insulator 5 from the same source which supplies compressed air to the column 15.

The switching apparatus of FIG. 1 operates in the following manner. When it is desired to open the cir cuit, valve is opened to admit compressed air from tank 1 to the interior of casing 9. Piston-cylinder valve unit 8 functions first to admit compressed air from casing 9 into insulator 3 at inlet 12 and elect disconnection of contacts 3a, 3b. This forces the current which had been passing through these contacts when closed to take a parallel path through the low-ohmic resistance 6. Piston-cylinder valve unit 7 then functions to admit compressed air from casing 9 into insulator 2 at inlet 11 and effect discon` nection of contacts 2a, 2lb which serves to interrupt the current tlowing through resistance 6. After

contacts

2a, 2b have been opened, compressed air is admitted through insulator column 14 into insulator S to effect separation of the voltage disconnecting

contacts

5a, 5b. These latter contacts are maintained in their open state by maintaining the air pressure within the column 14 and insulator 5 for so long as it is desired to hold the circuit open at the switching apparatus. However, as soon as the

contacts

5a, 5b have been separated, the power interrupting switching points at

contact sets

2a, 2b and 3a, 3b are reclosed. This is effected by means of the leakage of compressed air through small ports in the piston elements of the piston-cylinder

type valve units

7 and 8 so as to ultimately equalize the air pressures on opposite faces of these piston elements whereupon the restoring forces in springs 16 which are located in the cylinders serve to gradually reclose the entrances 11 and 12 to the insulators 2' and 3' and shut oft the supply of compressed air thereto. A spring loaded blow-off valve structure 13 is located at the outer end of insulator 2 and a similar valve structure is associated with the upper end of insulator 3. These valves which are structurally united with the movable contacts 2b, 3b are loaded by their springsl in such manner as to close ott the ends of their respectively associated insulators 2', 3 and also move the contacts 2b, 3b into engagement with their respective

stationary contacts

2a, 3a. When the

valves

7, 8 open to admit compressed air into insulators 2', 3', the contacts 2b, 3b are separated from their

respective contacts

2a, 3a and the valves 13 open against the counter action of their loading springs to discharge the compressed air from the insulators. When valves 7, S re-close, the blow-cti valves CII di. 13 also re-close to close ofi the ends of the insulators and re-close the

Contact sets

2a, 2b and 3a, 3b.

The necessary electrical series connection between the

stationary contacts

2a and 3a can be made through the wall of the auxiliary tank ll which can be made from electrically conductive material. The connection from contact 3b to contact 5b is made by suitable means, not shown in detail.

The embodiment which has been described can constitute one structural switch unit of the entire switch assembly. Another such structural switch unit containing two power interrupting switching points and a voltage disconnecting point can be arranged to the left of the one shown in FIG. l, i.e. as a mirror image of the first one, in which event the voltage disconnecting point could be branched ofI from the same supporting insulator column 14. Alternately a second such structural switch unit could be arranged in the same direction as the one depicted in FIG. 1.

FIG. 2 illustrates a somewhat diterent embodiment wherein the power interrupting switching point (

contact set

2a and 2b within its hollow insulator structure) is located atop the low-ohmic resistance unit which, in this embodiment takes the form of a tubular member 6' supported vertically upon the auxiliary tank 1 and which may be surrounded by an auxiliary hollow insulator. The other power interrupting switching point (Contact set 3a and 3b within its hollow insulator structure) is mounted vertically and directly upon the auxiliary tank 1 parallel with tubular member 6'. The voltage disconnecting switching point (contact set 5a and 5b within its hollow insulator structure is likewise mounted directly upon tank 1 and is disposed in a horizontal attitude laterally of the tank.

in this embodiment, the casing 9 located within tank 1 houses the main control valves '7' and 8. These are similar in structure to the

valves

7, 8 of the FIG. l embodiment but are actuated, not by compressed air admitted from tank 1 as in FIG. l, but rather by compressed air supplied to the casing 9 from a vertical, hollow supporting insulator 14 which serves as one support for the tank 1'. Air from casing 9 is also fed through the horizontal insulator of the voltage disconnecting switching point (contact set 5a', Sb) to separate these contacts after the power interrupting switching points (contact sets 2a', 2b' and Sa') have been opened.

A second hollow supporting insulator 15 is disposed parallel with insulator -14 and serves as the other support for tank 1. Compressed air which feeds upwardly through insulator 15 is admitted to the interior of tank i1 and the latter serves as the supply source of compressed air which feeds thnough inlets 11 and 12' when the control valves 7' and `t5' are opened to thus etect disengagement between the power interrupting switching points 2a', 2b and 3a', 3b. It will be noted that the compressed air which is used to open the contacts 2a', 2b must first pass upwardly through the tubular low-ohmic resistance unit 6' to thus cool the same. Moreover, the longer air path established by this resistance uni-t 6 provides the necessary delay in switching sequence so that the contacts 2a', 2b open subsequently to the opening of contacts 3a', 3b. As with the FIG. 1 embodiment, a high-ohmic resistance unit 4 can be connected in parallel with the contacts 2a', -Zb for potential control.

The manner in which the embodiment of FIG. 2 operates is the same as that of FIG. 1. The advantages of the iFIGS. 1 and 2 embodiments are that the two power interrupting switching points can be supplied with compressed air together from a common auxiliary tank, and that the assembly can be effected in a space-saving manner. Moreover, it is possible to combine any desired number of switch units, depending upon the voltage to which the switching apparatus is applied and the circuit breaking capacity desired. Several switch units can be omitted if the circuit breaking capacity is to be lower. It is even possible to eliminate one power interrupting switching point completely without changing the construction of the individual elements.

The switching for multiple break switches in accordance with the invention can be etected in any desired order and direction. yFor example, as shown in the circuit diagram of FIG. 3, the number of necessary resistances can be reduced and power interrupting switching points of different switch units can be connected together in such manner as to be paralleled by a common low-ohrnic resistance unit. Thus the complete switching assembly is comprised of two series connected switching units, each such unit comprising a series arrangement of two power interrupting switching points 2", 3 and a voltage disconnecting switching point l5". However, only one low-ohmic resistance unit 6 is used and this is connected in parallel with the two series connected switching points 3".

FIGS. 4 and 5 illustrate embodiments wherein the main control valves 17 are arranged on the exhaust side of the power interrupting switching points and separation of the contacts at the power interrupting switching points is effected not by introducing compressed air from an auxiliary tank to the interior of the insulators containing these switching points but rather by permitting compressed air which has been stored in these insulators and auxiliary tank to be discharged therefrom.

With reference now to FIG. 4 it will be seen that the auxiliary tank I1 is carried on the top of two, parallel spaced hollow insulator columns '14 and 15". Upstanding upon the tank 1 is a tubular insulator -21 which contains a set of power interrupting switching contacts 3a, 3b, and mounted at the top of insulator =21 is a housing 19 provided with an exhaust outlet 19a which is controlled by a valve 17. The lower end of insulator 21 communicates with the interior of tank 1 and the upper end of insulator 21 communicates with the interior of housing 19. The movable contacts 3b is loaded by a spring Z3 into engagement with the stationary contacts 3a, and the lov/ohmic resistance 6 is connected in parallel with the contacts 3a, 3b".

Upstanding upon the housing 19 is another tubular insulator 21 which contains the other set of power interrupting switching contacts 2a., 2b, and mounted upon the top of insulator 21 is a second housing 19' provided with an exhaust outlet 19a which is controlled by valve 17. The lower end of insulator 21 is placed in communication with the upper end ot insulator -21 by means of a duct 24, and the upper end of insulator 21 communicates with housing 19. The movable contacts 2b is loaded by a spring into engagement with the stationary contact 2a".

Also upstanding upon the tank `1" and spaced parallel with the

insulators

21, 21 is another tubular insulator 26 which serves as a support for the

piston elements

22, 22 of the control valves 17, 17 and their cylinders and also as a duct for the passage of compressed air from the upper end of support insulator 14 to actuate those piston elements. It will also be noted that compressed air from insulator 14 passes into the tubular insulator element which contains the contacts 5a", 5b of the voltage disconnecting switching point. As with the other arrangements, the two power interrupting switching points represented by

contact sets

2a, 2b" and 3a, 3b, and the contact set 5a, Sb are all connected electrically in series.

The arrangement shown in FIG. 4 operates in the following manner.

When all of the switch contacts are closed, compressed air fills the

insulators

21, 21 and the housings 19, 19'. To ei'ect an opening of the switch, compressed air is admitted through insulator 14" into the tubular insulator 26. When this air reaches the rst piston 22 it shifts the latter to the right thus opening the outlet 19a and permitting the compressed air stored in housing 19 to flow outward to atmosphere which results in an upward movement of contact 3b so as to disengage it from contact 3u. The current which had then been flowing through CTL these contacts is thus shunted over to flow through the low-ohmic resistance 6". By this time, the compressed air ilowing upward through insulator 26 past the branch off point to piston 22 will reach the other piston 22 to thus shift it to the right and thus open outlet 19a' which permits the compressed air stored in housing 19' to how outward to atmosphere and eiect an upward movement of Contact 2b so as to disengage it from contact 2a, AS these latter two contacts separate, the current which had been flowing through resistance 6" is interrupted. The flow of compressed air into the insulator containing the voltage disconnecting switching point (contacts Sa, `5b) then causes these latter contacts to separate, and they remain separated so as to effect the desired voltage isolation so long as the switch is in its disconnected state. However, the

pistons

22, 22 are provided with small orifices which ultimately serve to equalize the iluid pressure on opposite faces of the pistons so that the latter are moved back to the left by the action of their respective biasing springs 27, 27 and re-close the housing outlets 19a and 19a thereby resulting in a re-closing of the switch contacts 2a", 2b and 3a, 3b, and reestablishing the air pressure within the

housings

19, 19 and

insulators

21, 21. When it is desired to reclose the switch, air is vented from insulator 14 t0 atmosphere which results in a re-closing action of the conitacts Sa, 5b and a venting of air from the tubular insulator 26.

The series electrical connections between the power interrupting switching points and the voltage disconnecting switching point can be etlected by making the housing `19, 19 and auxiliary tank 1 from an electrically conductive material. Thus the circuit through the switch can be traced from housing 19 to contact 2b" to contact 2a. to housing 19 to contact 3b to contact 3a to tank 1" t0 contact 5b to contact 5a".

The arrangement illustrated in FlG. 5 is similar to that of FIG. 4 in that operation of the switch is eiiected by exhausting compressed air from the chamber containing the power interrupting switching contacts and hence similar parts have been given the same reference numerals. However, instead of separate valves for controlling the exhaust from the respective power interrupting switching points, only one control valve structure is used. With reference now to FiG. 5 it will be seen that a tubular insulator 255 upstands upon the auxiliary compressed air tank 1" which is supported by the two, parallel spaced

tubular insulators

14 and 15". The insulator 2S is lined with a sleeve 29 of conductive material which terminates in the stationary contact 3a" located within one end portion of housing 19 which is supported by the insulator Ztl. The opposite end of the vertically disposed housing 19 supports an insulator 30 through which extends the stationary contact structure Za" that is co-axial with the contact 361'".

The movable contacts 215'" and 3b" are carried by

pistons

31, 32 respectively which operate vertically in a common cylinder 33 located within housing 19, these pistons being biased in opposite vertical directions by a loading spring 3d disposed between the pistons so as to normally cause the sets of contacts controlled respectively thereby to be engaged.

Also upstanding upon tank 1 parallel with insulator 2S is another tubular insulator- 35 through which air is conducted from insulator 14 up to the piston 22 of control valve 17 which controls the opening and closing of the compressed air exhaust outlet 19a from housing 19". Operation of the switch of this embodiment is essentially the same as in the embodiment of FIG. 4. Cornpressed air introduced through insulator 3S causes piston 22 to move to the right thus opening the outlet 19a" and venting the compressed air stored in housing 1% to atmosphere. This also effects a venting of compressed air from the interior of cylinder 33 which establishes a temporary pressure differential on

pistons

31, 32 causing them to move towards each other in succession so as to disengage contacts Sb" and 211'" from their respective stationary contacts in that order. The voltage disconnecting switching contacts Sa and 5b then open. Valve piston 22 then shifts to the left to re-close the outlet 19a" whereupon the contact sets 2517", 2b and 3a", 311'" re-close as the pressure of the air rebuilds within housing 19 and interior of cylinder 33.

The series connection of the various switching elements in FIG. 5 can be traced from contact 251'" to contact 2b" to piston 31 and cylinder 33 which are made of electrically conductive material to conductive piston 32 to contact 3b" to contact 3a" to electrically conductive sleeve 29, to electrically conductive tank 1 to contact 5b to contact 5a.

I claim:

1. A compressed air actuated switch unit comprising a main tank containing compressed air, first and second hollow insulator columns upstanding upon said main tank, an auxiliary compressed air tank supported atop said iirst hollow insulator for receiving compressed air from said main tank, a rst hollow insulator extending laterally from said auxiliary compressed air tank and containing a pair of compressed air actuated power interrupting switching contacts, a second hollow insulator extending upwardly from said auxiliary compressed air tank and containing a second pair of compressed air actuated power interrupting switching contacts, valve means within said auxiliary compressed air tank for controlling ow of compressed air to said tirst and second hollow insulators for actuating said first and second pairs of switching contacts, a third hollow insulator mounted on said second insulator column and containing a pair of compressed air actuated voltage disconnecting switching contacts, one of said voltage disconnecting contacts being connected to one of the power interrupting contacts of said second pair of such contacts, said irst and second pairs of power interrupting switching contacts being connected in series, a low ohmic resistance connected in parallel with said second pair of power interrupting switching contacts, and a high ohmic resistance connected in parallel with said first pair of power interrupting switching contacts.

2. A compressed air actuated switch unit comprising a main tank containing compressed air, a pair of hollow insulator columns upstanding on said main tank, an auxiliary compressed air tank supported at the top of said insulator columns, the interior of said auxiliary tank being in communication with said main tank through one of said insulator columns, a low ohmic resistance em- 1bodied in a lirst hollow insulator upstanding upon said auxiliary tank, a second hollow insulator upstanding upon said irst hollow insulator and containing a tirst pair of compressed air actuated power interrupting switching contacts paralleled by a high ohmic resistance, a third hollow insulator upstanding upon said auxiliary tank and containing a second pair of compressed air actuated power' interrupting switching contacts and which are connected in parallel with said low ohrnic resistance, a fourth hollow insulator extending laterally from said auxiliary tank and containing a pair of compressed air actuated voltage disconnecting contacts, all of said pairs of contacts being connected together in series, and compressed air actuated valve means included within said auxiliary tank controlling tlow of compressed air into said first, second and third hollow insulators, compressed air to said control valves and to said fourth hollow insulator being delivered thereto from said main tank through the other of said insulator columns.

References Cited in the tile of this patent UNITED STATES PATENTS 2,453,555 Thommen Nov. 9, 1948 2,599,100 Forwald lune 3, 1952 2,873,331 Thommen Feb. 10, 1959 2,894,100 Hoffmann July 7, 1959 2,969,446 FOrwald lan. 24, l96l FOREIGN PATENTS 568,382 Belgium lune 30, 1958 913,482 France May 27, 1946

Claims

1. A COMPRESSED AIR ACTUATED SWITCH UNIT COMPRISING A MAIN TANK CONTAINING COMPRESSED AIR, FIRST AND SECOND HOLLOW INSULATOR COLUMNS UPSTANDING UPON SAID MAIN TANK, AN AUXILLIARY COMPRESSED AIR TANK SUPPORTED A TOP SAID FIRST HOLLOW INSULATOR FOR RECEIVING COMPRESSED AIR FROM SAID MAIN TANK, A FIRST HOLLOW INSULATOR EXTENDING LATERALLY FROM SAID AUXILLIARY COMPRESSED AIR TANK AND CONTAINING A PAIR OF COMPRESSED AIR ACTUATED POWER INTERRUPTING SWITCHING CONTACTS, A SECOND HOLLOW INSULATOR EXTENDING UPWARDLY FROM SAID AUXILLIARY COMPRESSED AIR TANK AND CONTAINING A SECOND PAIR OF COMPRESSED AIR ACTUATED POWER INTERRUPTING SWITCHING CONTACTS, VALVE MEANS WITHIN SAID AUXILIARY COMPRESSED AIR TANK FOR CONTROLLING FLOW OF COMPRESSED AIR TO SAID FIRST AND SECOND HOLLOW INSULATORS FOR ACTUATING SAID FIRST AND SECOND PAIRS OF SWITCHING CONTACTS, A THIRD HOLLOW INSULATOR MOUNTED ON SAID SECOND INSULATOR COLUMN AND CONTAINING A PAIR OF COMPRESSED AIR ACTUATED VOLTAGE DISCONNECTING SWITCHING CONTACTS, ONE OF SAID VOLTAGE DISCONNECTING CONTACTS BEING CONNECTED TO ONE OF THE POWER INTERRUPTING CONTACTS OF SAID SECOND PAIR OF SUCH CONTACTS, SAID FIRST AND SECOND PAIRS OF POWER INTERRUPTING SWITCHING CONTACTS BEING CONNECTED IN SERIES, A LOW OHMIC RESISTANCE CONNECTED IN PARALLEL WITH SAID SECOND PAIR OF POWER INTERRUPTING SWITCHING CONTACTS, AND A HIGH OHMIC RESISTANCE CONNECTED IN PARALLEL WITH SAID FIRST PAIR OF POWER INTERRUPTING SWITCHING CONTACTS.